1. Field of the Invention
The present invention relates to a tracking control unit for an optical disk drive capable of controlling tracking using an actuator of a fine-and-coarse driving integrated type.
2. Description of the Related Art
For realizing cost reduction for an optical disk drive, a method using the thrust of the same coil to control both tracking and access is effective. Specifically, as disclosed in Japanese Unexamined Patent Publication No. 63-224037, one tracking actuator is used for both fine driving and coarse driving instead of including a fine-driving actuator (designed for controlling tracking and movable in a narrow range) and a coarse-driving actuator (designed for controlling access and movable in a wide range).
For materializing such an actuator of a fine-and-coarse driving integrated type, a bearing may be placed between guide rails for supporting a carriage having a focus actuator and reflection mirror mounted thereon. In this case, there arises problems that a movable portion becomes large in size, a weight increases, and a resonant frequency for a high-order mode of resonance falls accordingly. A support for the carriage is therefore designed to be of a sliding type (sliding bearing) in practice.
When a bearing is of the sliding type, every time the carriage moves, frictional force works on the carriage and guide rails. When frictional force is present, a phenomenon that a deviation remains with a control residual derived from frictional force and driving force generated by a tracking servo system against the control residual balanced takes place. The residual is referred to as an individual frictional distance.
When tracking is controlled in an optical disk drive, a control residual corresponding to the individual frictional distance is observed at two points during one rotation of a disk depending on an eccentric state of a track.
To be more specific, as shown in FIG. 17, since information tracks on an optical disk are generally eccentric, when the disk is rotated, a certain information track is seen shifting in a direction substantially orthogonal to the tracks according to a rotational frequency ("eccentric position" in FIG. 17). Owing to tracking control, a light beam attempts to follow the eccentricity. As shown in "eccentric speed" in FIG. 17, when a moving speed is zero, that is, when a magnitude of eccentricity reaches peak, a state of static friction is attained. A control residual therefore increases transiently ("control residual" in FIG. 17). In some eccentric state, a residual may occur at many instants of time.
A control residual of a certain magnitude adversely affects the data writing/reading characteristic of an optical disk drive.
A way of reducing the residual is to increase a loop gain of a tracking control system. It is difficult for an actuator of a fine-and-coarse driving integrated type to set the resonant frequency for a high-order mode of resonance to a high frequency for structural reasons. Since it is hard to raise a frequency band to which servo system is sensitive, it is difficult to increase the loop gain.
By the way, for coping with a phenomenon that a residual resulting from tracking control increases transiently due to vibration or the like, a method in which when a window comparator or the like detects that the control residual becomes equal to or larger than a given value, a gain to be produced by a control system is increased is known. One approach has been disclosed in Japanese Unexamined Patent Publication No. 6-139588.
However, the approach described in the Japanese Unexamined Patent Publication No. 6-139588 is intended to reset a moving device for a short period of time when a relatively large residual such as a servo failure derived from external vibration or a flaw on a disk occurs or when a servo loop is disabled. It is difficult to suppress a relatively small residual such as a transient increase in control residual derived from friction which is critical for an actuator of a fine-and-coarse driving integrated type supported by a sliding bearing.
For suppressing a small residual, a threshold is lowered so that a servo system can sensitively react on the small residual. In the approach described in the Japanese Unexamined Patent Publication No. 6-139588, judgment is made from the level of a residual alone. In case a tracking control system oscillates due to an increase in gain, since a tracking error signal whose level gets higher because of oscillation is detected, the gain remains increased. Consequently, the oscillation may be maintained.
Furthermore, in general, the tracking control system is susceptible to the state of a disk employed. For example, the distortion of a tracking error signal occurring at a pre-pit area or the occurrence frequency of a control residual derived from the eccentricity of the disk depends greatly on the state of a disk. When an attempt is made to design the tracking control system so that it can tolerate such factors and be kept off malfunctions including oscillation, if the approach described in the Japanese Unexamined Patent Publication No. 6-139588 is adopted, since a threshold must be set to a large value, the ability to suppress a microscopic control residual such as a residual derived from friction hardly works.